Radioactive source



. topes.

,testing and thickness measurement.

non-destructive testing of materials, the principal source ment.

atet

2,830,190 Patented Apr. 8, 1958 ice 2,830,190 RADIOACTIVESOURCE WilliamKarp, Dedham, Mass.,.assignor to Tracerlab, Inc., Boston, Mass., a corporation of Massachusetts Application February 5,1954, Serial No. 408,345

7 Claims. (Cl. 250-106) This invention relates generally to radioactive sources and more particularly to an improved source holder and a method for the fabrication thereof which is particularly adaptable for certain artificially produced radioactive iso- Wide applications for radioactive sources exist in industry, among them being radiographic,non-destructive Heretofore, in the of radiation has been radium because of the desirable characteristics of its radiation. An early form of radium source consisted of a radium salt hermetically sealed in 2 of the solvent to leave a radioactive layer on the surface. This method is suitable where low-level sources are required, but in cases where sources of high level activity are needed, the amount of solution needed to deposit the desired amount of activity upon evaporation is so great as to render the method impractical and unworkable. Other prior art methods have taught the utilization of a precipitation step to separate the radioactive material from solution, but in general, no satisfactory technique has been advanced for the uniform application of, the

precipitate to the source mount.

It is an object of this invention, therefor, to provide a new and improved method for producing a radioactive source.

Another object of the invention is to provide a convenient and time-saving method for separating a radioactive material from solution and incorporating it in a leak-proof source.

Another object of the invention is to provide a method for producing a radioactive source in which there is a minimum of hazard to operating personnel.

A further object of the invention is to provide a radioactive source of a design such that the fabricationthereof can be carried out with remote handling equipment.

With the foregoing and other objects in view,the radioactive source of the present invention is featured by an a silver containenwhich in turn was enclosed in an aluminum alloy capsule. The capsule. was provided with an eyelet through which a string was passed to facilitate handling and positioning of the source. the handling problem has been eliminated to a large extent through the use of a two-part capsule, one part of aluminum and the other of a magnetic material, to permit handling withmagnetic remoteIhandling equip- A source of this type, and a representative handling tool are described in U. SQPatent No. 2,269,458, dated January 13, 1942.

With the availability of artificially produced radioisotopes, the use of radium in industrial applications has fallen off somewhat, and has been replaced to a large extent by isotopes such as cobalt-60, iridium-192 and cesium-137. A radiography source employing iridium is described in copending application S. N. 391,980, filed Y November 13, 1953, and assigned to the assignee of the .present application. which. may make it a valuable source for future industrial radiography. This isotope has an energy level of 0.67 million electron volts, thus bridging the gap between Cesium- 137 has many properties the energy levels of iridium-192 and cobalt-60, and permitting its use in the inspection of materials of intermediate thickness and density. In addition to its intermediate energy level, cesium-137 has a comparatively long half-life of 33 years, and while it is a pure beta emitter, it acquires its gamma ray emission from its daughter, barium-137, with which it is in secular equilibrium. Cesium-137 has the. further advantage over iridium-192 and cobalt-60 in that it is a fissionproduct rather than irradiated material, and should therefore ultimately be more readily. available and less expensive since it is a by-product of the nuclear reactor.

The cesiuml37 is normally obtainable from Oak Ridge (or other reactor). as cesium nitrate or cesium N chloride in solution, a form which cannot readily be used in the fabrication of a radiography source. It is desirable, in most cases, to employ the isotope invthe form of a salt, so that it can be readily sealed and mechanically fixed in place. Radioactive sources have previously been fabricated by the deposition of a radioactive salt on a suitable backing surface, but in general, the techniques used have been rather unsatisfactory. U. S. Patent No. 2,575,134. for example, .teaches the application of a radioactive;solutionto a desired surface, and evaporation More recently,

carried out with remote pipetting equipment safely positioned behind a suitable shielding wall. The radioactive slurry thus formed is transferred to the cup-like member mentioned above, and the solvent removed, leaving the salt in the cup-like member. The cup-like member is then transferred to the outer container and secured in place by a closure plug.

Other features, objects and advantages of the invention will be apparent from the following detailed description when taken in connection with the accompanying drawings in which:

Fig. lis an enlarged sectional view,'in elevation, of a preferred embodiment of the radioactive source of this invention; i a Fig. 2 is an elevation cross-section of a preferred apparatus for depositing the radioactive salt on the cup-like member of the source; and

Fig. 3 is an enlarged sectional view, in elevation, showing a preferred method of inserting the cup-like, memher into the outer container.

Referring to Fig. l of the drawings, thesource of the 10, preferably formed of a magnetic material, having a cylindrical bore 11, the open end 12 of which is threaded to receivea closure cap or plug 13. The closure plug 13 is fittedwith a centrally located lug 14 having an eyelet 15 to permit handling of the source with a string, wire or chain.

The active portion of the source assembly consists of a cup-like structure formed of a short length of tubing 16, preferably formed of stainless steel, joined to a disc 17 of porous stainless steel, in which is deposited a radioactive salt 18of cesium-137, or other suitable radioisotope. The significance of the porous stainless steel disc 17 and the manner in which the salt is deposited will be apparent from the discussion tofollow. The cup-like structure is placed in .inverted position within cylinder 19 so that self-absorption of the radiation by the source 3 from the isotope need pass only through the relatively thin closed end of the cylinder 10.

The source assembly is completed by a spacer plug 20 positioned between the cup-like structure and the closure plug. Plug 19 is formed with a central opening 21 t form a gas reservoir in the event that any gases are liberated from the source material, and is preferably made of aluminum for reasons which will appear later.

In the preparation of the source, the radioisotope, which by way of example, will be considered cesium-137, is obtained as cesium nitrate or cesium chloride in solution, a form which does not lend itself to direct utilization in a source of the industrial type. The salt of the isotope being more desirable, the active material is precipitated by addition of a suitable reagent to the solution. In the case of cesium nitrate, it has been found advantageous to precipitate the source material by the addition of chloroplatinic acid. Alternatively, precipitation may be accomplished by the addition of perchloric acid to form cesium perchlorate, or the addition of silicotungstic :acid to form cesium silicotungstate. Whatever the reagents used, the precipitation step is conducted with remote handling equipment with the precipitation vessel :arranged behind a suitable shielding wall to protect the operator. Such equipment is commercially available and permits the handling of reagents without physical contact with the chemical and at a distance where the hazard from radiation is relatively small.

Also by remote control, using elongated tongs or the like, the radioactive slurry formed by precipitation, is transferred to the cup-like structure described above. During this step, the cup is mounted in upright position, as shown in Fig. 2, on a funnel-like structure 22, to which is connected a vacuum pump (not shown). The porous stainless steel disc 17 functions similarly to a filter paper, the solvent being drawn through the disc while the radioactive precipitate is deposited in the volume defined by the tubing 16. Removal of the solvent in this manner promotes the uniform settling of the precipitate, a feature which is particularly advantageous when very thin active elements are desired. Transferral of radioactive slurry and removal of solvent is continued until the cup defined .by the wall of tube 16 (which may be of any desired length) is substantially filled.

After the source container is sutficiently full, a solution of sodium silicate is added to the top of the precipitate 18 and the source subjected to heat, for example, a heat lamp, to dry the precipitate and set the sodium silicate. The sodium silicate forms a thin film 19 over the source material preventing its disturbance or loss upon inversion and subsequent handling.

The source container is then transferred to the cylinder 1%) using a loading funnel 23 (Fig. 3) and remote handling tongs, and is positioned with the source material 18 directed downward as shown. The use of the loading funnel minimizes disturbance of the radioactive material during assembly, and eliminates contamination of the source body. The funnel 23 is formed of a tube of very thin material and is so designed that it can be lifted out when slight pressure is applied to the outer container 10. Plug 20 is then inserted into the source container, and plug 13 threaded into the open end, rigidly to secure all of the parts together.

After assembly in accordance with the foregoing procedure, all of which is done with remote handling equipment, the source is transferred to a lead shielding member having an opening therein to receive container 10. With the source shielded in a radial direction and axially downward, the cap 13 is puddled over with solder to hermetically seal the source and to prevent tampering with the cap which could result in accidental leakage of the source material. After soldering, the finished source is placed in a suitable shielded container for storage and shipment.

As was noted earlier, plug 20 is formed of aluminum and serves the two-fold purpose of (1) reacting with any chlorine that may be liberated from the Cs PtCl source material and (2) providing a space 21 for possible expansion of gases within the container 10 and eliminating the build-up of undesirable pressures.

The drawings of the source have been greatly enlarged for clarity, the actual overall size being of the order of /s" in length and A in diameter, the active volume of the radioactive salt being about X Ma" in size. It Will be understood, of course, that these dimensions are illustrative only, and that the shape and size of the active area will be determined by the application for which the source is intended. In the case of a radiography source, for example, it is desirable to provide a point source; i. e., of minimum area, whereas thickness gauge sources preferably are of relatively large active area.

While a preferred embodiment of the invention has been illustrated and described, it will be apparent that various modifications may be made therein without de parting from the spirit theerof, and it is therefore intended that the invention be limited only as such limitation occurs in the appended claims.

What is claimed is:

1. A radioactive source comprising a metallic cylindrical container having a closed end and an open end, a cylindrical cup-like container having its bottom formed of porous stainless steel, a deposit of radioactive salt in said cup-like container, said cup-like container being positioned within said outer container with said radioactive salt adjacent the closed end of said outer container, a closure cap hermetically sealed in the open end of said outer container, and a spacer plug formed of a material adapted to react with any gas which may be liberated from said radioactive salt disposed between said closure can and said cup-like container.

2. A radioactive source comprising a cylindrical outer container formed of a magnetic material and having a closed end and a threaded open end, a cylindrical cuplike container having a bottom formed of porous stainless steel, a deposit of radioactive salt in said cup-like container, a film of sealing material covering said deposit of radioactive salt, said cuplike container being disposed within said outer container with said radioactive salt adjacent the closed end of said outer container, a closure cap secured in the threaded open end of said outer container, and a spacer plug formed of a material adapted to react with any gas that may be liberated from said radioactive salt disposed between said closure cap and said cup-like container.

3. A radioactive source comprising a cylindrical outer container formed of a magnetic material and having a closed end and an open end, a cup-like container having 'its bottom formed of porous stainless steel, a deposit of radioactive salt filling said cup-like container, said cuplike container being disposed within said outer container with said radioactive salt disposed adjacent the closed end of said outer container, a closure cap hermetically sealed in the open end of said outer container, and a spacer plug having an opening therein disposed between said closure cap and said cup-like container, said spacer plug being formed of a material adapted to react with any gas that may be liberated from said radioactive salt and the opening therein being provided to prevent the build-up of gas pressure within said outer container.

4. In a process for preparing a radioactive source, the steps of precipitating a'radioactive salt from a radioactive solution by the addition of a suitable reagent, transferring the resulting radioactive slurry to a container having a bottom formed of porous stainless steel, removing the excess solvent by the application of suction to the underside of said container, and applying a film of sealing material over the top of the thus deposited salt.

A .n fiihod of preparing a radioactive source comprising the steps of precipitating a radioactive salt from a radioactive solution by the addition of a suitable reagent, transferring the resulting radioactive slurry to a container having a bottom formed of porous stainless steel, removing the excess solvent by the application of suction to the underside of said container, adding a solution of sodium silicate to the top of the thus deposited salt, and heating said container to set said sodium -silicate and to dry said salt.

6. A method of preparing a radioactive source comprising the steps of precipitating a radioactive salt from a radioactive solution by the addition of a suitable reagent, transferring the resulting radioactive slurry to a container having a bottom formed of porous stainless steel, removing the excess solvent by the application of suction to the underside of said container, adding a solution of sodium silicate to the top of the thus deposited salt, heating said container to set said sodium silicate and to dry said salt, and hermetically sealing said container in an oute container formed of magnetic material.

7. A method of preparing a radioactive source com prising precipitating an insoluble salt from a solution of radioactive cesium by the addition of a suitable acid, transferring the precipitate to a container having a bottom formed of porous stainless steel, removing the excess solvent by the application of suction to the underside of said container, adding a solution of sodium silicate to the top of the deposited salt, and applying heat to said container to set said sodium silicate and to dry said salt.

References Cited in the file of this patent UNITED STATES PATENTS 789,812 Kunz May 16, 1905 1,718,626 Bleecker June 25, 1929 2,326,631 Fischer Aug. 10, 1943 2,575,134 Schultz et al. Nov. 13, 1951 2,592,115 Carroll Apr. 8, 1952 

3. A RADIOACTIVE SOURCE COMPRISING A CYLINDRICAL OUTER CONTAINER FORMED OF A MAGNETIC MATERIAL AND HAVING A CLOSED END AND AN OPEN END, A CUP-LIKE CONTAINER HAVING ITS BOTTOM FORMED OF POROUS STAINLESS STEEL, A DEPOSIT OF RADIOACTIVE SALT FILLING SAID CUP-LIKE CONTAINER, SAID CUPLIKE CONTAINER BEING DISPOSED WITHIN SAID OUTER CONTAINER WITH SAID RADIOACTIVE SALT DISPOSED ADJACENT THE CLOSED END OF SAID OUTER CONTAINER, A CLOSURE CAP HERMETICALLY SEALED IN THE OPEN END OF SAID OUTER CONTAINER, AND A SPACER PLUG HAVING AN OPENING THEREIN DISPOSED BETWEEN SAID CLOSURE CAP AND SAID CUP-LIKE CONTAINER, SAID SPACER PLUG BEING FORMED OF A MATERIAL ADAPTED TO REACT WITH ANY GAS THAT MAY BE LIBERATED FROM SAID RADIOACTIVE SALT AND THE OPENING THEREIN BEING PROVIDED TO PREVENT THE BUILD-UP OF GAS PRESSURE WITHIN SAID OUTER CONTAINER. 